One of the main aims of packet networks and frame relay systems is to maximize the data throughput of a system given a limited bandwidth communication link. In the case of speech information, conventional code division multiple access (CDMA) cellular systems take advantage of the statistical nature of speech to increase the number of channels handled by a given data "pipe". This is particularly the case with CDMA systems where speech coding algorithms in voice coders (vocoders) generate variable speech packet sizes that are directly related to the amount of speech activity at a given time.
As the number of channels is increased, there is an increased occurrence of packet blocking where the data rate of the link is exceeded by the short-term requirements of the speech (voice) packets. Therefore, the number of channels is typically set at a level where these collisions occur at a statistically low level. When such collisions do occur, speech packets are typically thrown away or bits within a speech packet have to be thrown away to reduce the packet size. This inevitably results in reduced voice quality due mainly to a loss of relevant speech information and to a subsequent divergence of the voice encoder's filter state to the voice decoder's filter state. For example, where a base site controller (BSC) encodes speech in its transcoder as received from a mobile switching center, a mobile subscriber unit receiving the voice has a decoder filter whose states should be similar to the state of the encoding filter to maintain a proper historical record for use by the filters to properly encode and decode voice signals. Where the filter states differ between an encoder and the corresponding decoder, a loss in voice quality can occur over time.
In a conventional code division multiple access cellular radio telephone system, the conventional limited bandwidth communication link which passes the transcoded information is a T1 Megastream link, which, assuming a data traffic transfer rate of 16 kilobits/sec, yields 96 channels. Also in conventional CDMA cellular radiotelephone systems, a centralized base site controller uses the transcoder to transform data at a higher rate into data at a lower rate (and subsequently smaller bandwidth). The lower rate data is called transcoded, or rate adapted data. The data is typically coded in traffic channel packets and are communicated to and from a network arbitor which arbitrates among the traffic channels to select which packets are communicated at which time sequences and at which frequency. A network arbitor interfaces with a network interface to communicate the information coming from the transcoder to a base station. The base station then transmits and receives radio signals with a mobile subscriber unit. However, the network arbitor does not typically communicate back to the variable rate vocoders to vary encoding rates or inform the variable rate vocoder that speech packets were dropped to avoid bandwidth bottlenecks.
Forms of increasing system capacity by reducing speech encoder rates-are known. For example, U.S. patent application Ser. No. 08/575,450 filed Dec. 20,1995 entitled "Method and Apparatus for Controlling Encoding Rate in a Communication System" assigned to the instant assignee, proposes a system that identifies specific mobile subscriber units as candidates for encoding rate reduction based on link related characteristics from a group of mobile subscriber units. The link related characteristics, include for example the handoff state of the mobile, the location of the mobile with respect to a serving base station or adjacent base stations, transmission characteristics of the mobile and the amount of noise experienced by the mobile. However, the network arbitor in such a system also does not typically communicate back to the variable rate vocoders to vary encoding rates or inform the variable rate vocoder that speech packets were dropped to avoid bandwidth bottlenecks.
Consequently, a need exists for an apparatus and method which facilitates a maximization of the available bandwidth over a communication link while also maintaining a suitable voice quality level during high capacity link usage. It would be advantageous if such a system could selectively control the variable rate vocoders to change their encoding rate when a required bandwidth exceeded an available bandwidth.